Hollow waveguide sector antenna

ABSTRACT

A hollow waveguide group antenna comprises a hollow waveguide extending in a direction in space and a plurality of chambers, each of which has a sending/receiving slit and is coupled to the hollow waveguide by a coupling slit. The sending/receiving slits are distributed at a fixed distance from each other, and the distribution of the coupling slits in the direction in space at the transversal hollow waveguide is selected differently from the distribution of the sending/receiving slits such that a wave propagating at the working frequency excites the sending/receiving slits with amplitudes and phases suitable for realizing a sector direction characteristic. The fixed distance is approximately 0.5λ 0  for 90° sector direction characteristic and approximately 0.64 π 0  for a 45° sector direction characteristic.

The present invention relates to a sector antenna.

Performance requirements for sector antennas for wireless transmissionare very high. These are uniform coverage of a certain range, e.g. a 90°sector, in the horizontal plane with a strong intensity decrease ofsidelobes, and a highly directive, zero-free characteristic for thevertical plane. From H. Ansorgen, M. Guttenberger, K.-H. Mierzwiak, U.Oehler, H. Tell, “Antenna solutions for point to multi-point radiosystems” ECRR, Bologna 1996 and M. Guttenberger, H. Tell, U. Oehler,“Microstrip-Gruppenantennen mit scharf sektorisierenden Eigenschaftenals Zentralstationsantennen fuar Punkt zu Multipunkt Systeme”, ITGFachtagung Antennen, Muinchen, 1998, it is known to realize such sectorantennas in strip-line technique.

A general problem of such conventional sector antennas is aninsufficient suppression of cross polarization.

In order to realize a desired directional characteristic of such a groupantenna, its individual radiating elements must be excited withdifferent excitation coefficients. These excitation coefficients arecomplex, i.e. they are characterized by magnitude and phase. Methods forcalculating them are known. The excitation is achieved using adistributing network that distributes a transmission signal fed into itsinput to the individual radiating elements. The assigned excitationcoefficients are defined by the structure of the distributing network.

Distributing networks in strip-line technique are disadvantageous due totheir losses. These losses increase strongly with increasing operatingfrequencies of the distributing network, so that in particular at highoperating frequencies, there is a need for group antennas with reducedloss. Such group antennas may be realized in hollow waveguide technique.

A problem with the design of hollow waveguide group antennas is that forrealizing a desired sector characteristic, specific small distances arenecessary between adjacent radiating elements, which radiate atessentially opposite phases. E.g. for a 90° sector characteristic, thisdistance is approximately 0.5 λ₀, wherein λ₀ is the free spacewavelength of a wave emitted by the antenna. The length λ_(H) of a waveof given frequency in a hollow waveguide of finite cross section isalways greater than its wavelength λ₀ in free space; it convergestowards the free space value if the width of the hollow waveguideapproaches infinity. With a group antenna whose radiating elements areapertures in a hollow waveguide wall, a satisfying sector characteristicmight theoretically be achieved if an extremely wide hollow waveguide isused. However, this is not a technically practical solution.

A group antenna according to the preamble portion of claim 1 is knownfrom U.S. Pat. No. 6,127,985.

This prior art group antenna is formed of a plurality of layers. A firstsuch layer comprises a two-dimensional arrangement of chambers, each ofwhich has a sending/receiving slit and a coupling slit, respectively, atopposite sides thereof. The coupling slits of several chambers jointlylead into a transversal hollow waveguide extending in a second layer.The distance of the coupling slits along the transversal hollowwaveguide is selected so that all coupling slots are excited at equalphase, i.e. the distance of the coupling slits corresponds to thewavelength in the transversal hollow waveguide at a resonance frequencyof the antenna. Since the chambers of this prior art antenna have thesame geometry, the sending/receiving slits of all chambers radiate atequal phases. Thus, with a large number of slits, a strong collimationof the main lobe of the radiation diagram can be realized. There is nofilling up of zeros of the direction characteristic. A sectorcharacteristic cannot be realized with this prior art antenna.

The object of the present invention is to provide a compact groupantenna with sector characteristic having low losses even at highfrequencies.

The object is achieved by a group antenna having the features of claim1.

Besides low loss, this group antenna has the additional advantage of areduced cross polarization in comparison to stripline antennas.

The proposed solution relies on the conception that by sandwichingchambers between sending/receiving slits of a group antenna and a hollowwaveguide, here referred to as transversal hollow waveguide, whichjointly supplies the sending/receiving slits, it is possible to excitethe sending/receiving slits with appropriate phases and amplitudes for asector characteristic by selecting the arrangement of the coupling slitsat the transversal hollow waveguide—at variance from the arrangement ofthe sending/receiving slits at an outer side of the antenna—such thatthe coupling slits come to lie at places of the transversal waveguide atwhich fields with appropriate amplitude and phase relationships may becoupled out.

The transversal hollow waveguide has a short-circuit at at least one endthereof, so as to reflect waves propagating in the transversal hollowwaveguide. The distance of this short-circuit from the closest adjacentcoupling slit preferably amounts to approximately half of the hollowwaveguide wavelength of a wave propagating in the transversal hollowwaveguide at the operating frequency. Thus, a highest possible intensityof this wave at the location of this coupling slit is achieved.

The sending/receiving slits are preferably oriented transversally to thefirst spatial direction, i.e. the longitudinal direction of thetransversal hollow waveguide. Thus it is possible give the slits alength of approximately λ₀/2, so that they are resonant at the workingfrequency of the antenna or close to this frequency.

Simulation analyses have shown that a distance that is slightly largerthan half of the free space wavelength, particularly in the rangebetween 0.51 and 0.55× the free space wavelength, is advantageous forrealizing a 90° sector characteristic.

For a 45° sector characteristic, a distance between 0.58 and 0.63×,preferably of approximately 0.62× the free space wavelength, isappropriate.

According to a preferred embodiment, the arrangement of the couplingslits is mirror symmetric with respect to a symmetry plane orientedtransversally to the first spatial direction, and the transversal hollowwaveguide has an excitation aperture intersecting the symmetry plane. Acentered excitation of the transversal hollow waveguide by such anaperture has the advantage, with respect to excitation at an end of thehollow waveguide, that the maximum difference between the phase valueswith which a wave propagating in the transversal hollow waveguideappears at the coupling slits is only half as large under centeredexcitation than under end excitation, so that a larger bandwidth of theantenna can be achieved.

Of course, in case of centered excitation, it is appropriate toterminate both ends of the transversal hollow waveguide by a shortcircuit. The number of coupling slits of the transversal hollowwaveguide is preferably between 4 and 6. It is assumed that with largernumbers of coupling slits and chambers connected thereto, group antennaswith an excellent sector characteristic may be realized, but it has beenfound that with four coupling slits, very good results can already beachieved, so that more effort is not necessary.

Due to the centered excitation of the transversal hollow waveguide, thephase of chambers adjacent to the symmetry plane is always the same,regardless of the distance of the coupling slits of these chambers fromthe symmetry plane. Therefore, this distance may be varied in order toinfluence the resonance frequency of the transversal hollow waveguide orto optimize the amplitude/phase relationship between the sending slitsadjacent to the symmetry plane and the remaining sending slits. Adistance between the symmetry plane and the adjacent coupling slits ofapproximately one fourth of the hollow waveguide wavelength has beenfound to be appropriate.

For adapting amplitudes and phases, it is also possible to adapt thedistance between a coupling slit adjacent to the symmetry plane and acoupling slit adjacent to the short-circuit. Here, a value ofapproximately 0.3 hollow waveguide wavelengths has been found to beappropriate.

With the group antenna described above, a sector characteristic in afirst plane, in a practical application preferably the horizontal plane,may be realized. In order to achieve a collimation in a planeperpendicular thereto, i.e. preferably in the vertical plane, it ispreferred to employ an arrangement of several such group antennas, inwhich the transversal hollow waveguides of the group antennas areparallel and which may be referred to as a “two-dimensional groupantenna”.

In order to jointly feed the group antennas of the two-dimensional groupantenna, it is preferred that each transversal hollow waveguide has anexcitation aperture leading to a hollow waveguide, which is common toseveral transversal waveguides.

In order to achieve a collimation in the second plane, it is desirablethat adjacent transversal hollow waveguides are excited at approximatelyequal phases by a wave propagating in the common waveguide at theworking frequency, in order to obtain approximately equal phases betweenthe sending/receiving slits corresponding to these transversal hollowwaveguides, too. Deviations from the exact identity of the phases aredesirable in order to prevent a decrease to zero between adjacentmaximums of the direction characteristic.

According to a first embodiment, the common hollow waveguide may be alongitudinal hollow waveguide extending straightly in a second directionin space.

If this longitudinal hollow waveguide is a rectangular hollow waveguide,the width a of its sidewall in which the excitation apertures are formedis preferably given by${a = \frac{\lambda_{0}}{2\sqrt{1 - \frac{\lambda_{0}^{2}}{4d^{2}}}}},$wherein λ₀ is the free space wavelength of a working frequency of thegroup antenna and d is the distance between adjacent excitationapertures of the longitudinal hollow waveguide. In this way, a phasedifference of π between two adjacent excitation apertures can berealized for the wave propagating inside the longitudinal hollowwaveguide at the working frequency.

In order to be able to couple waves at equal phases—except forcorrection terms—into the transversal hollow waveguides at allexcitation apertures, it is desirable that mutually adjacent excitationapertures have coupling coefficients with opposite signs. For thispurpose, mutually adjacent excitation apertures are located atalternating sides of the center plane of the longitudinal hollowwaveguide. A fine tuning of the phase of the coupled transversalwaveguide waves is possible by an appropriate choice of a rotation angleof each excitation aperture with respect to the center plane. Such arotation also has an influence on the amplitude of the coupledtransversal waveguide wave, but this influence can be compensated by anappropriate choice of the lateral deviation of the excitation aperturefrom the center plane.

In order to avoid perturbations of the coupling by reflections at an endof the longitudinal hollow waveguide, it is preferred to locate ashort-circuited end of the hollow waveguide in a distance d/2 from theexcitation aperture adjacent to it.

According to a second embodiment of the invention, the first hollowwaveguide is formed as a tree structure having a trunk and a pluralityof branches, each of which connects the trunk to one of the excitationapertures. The individual branches may easily be assigned differentlengths and, hence, phase corrections. Further, bifurcations may beformed asymmetrically, in order to achieve a desired non-uniform powerdistribution to the individual branches as required in order to obtainamplitude and phase conditions at the radiating elements as required fora zero-free collimation in the second plane. This embodiment has theadvantage that the length of the branches must not differ from eachother by more than λ_(H), wherein λ_(H) is the wavelength at the workingfrequency of the group antenna inside the tree structure. I.e. if a wavepropagating within the tree structure deviates from this workingfrequency, the deviations cannot produce accumulating phase errors thatoccur in case of the longitudinal hollow waveguide, so that, compared tothis solution, a much larger bandwidth of the group antenna can beachieved.

The tree structure preferably has two main branches issuing from acommon trunk and extending at opposite sides of a plane extendingthrough the excitation apertures, wherein the excitation apertures ofmutually adjacent transversal hollow waveguides are each connected todifferent one of these main branches. This structure makes it very easyto tune deviations of the individual transversal hollow waveguides froma common phase that are necessary in order to avoid zeros of thedirection characteristic in the second plane, by choosing the hollowwaveguide length between the trunk and each individual excitationaperture.

In order to optimize the direction characteristic in the second plane,it is desirable to be able to excite the various transversal hollowwaveguides at different amplitudes. For this purpose, the branches ofthe tree structure leading to the excitation apertures preferably havedifferent power levels.

The different power levels are preferably realized at bifurcations, e.g.T- or Y-sections of the tree structure by conferring different crosssections on portions of such a bifurcation that lead to differentapertures. Specifically, these different cross sections may be obtainedby a tongue extending asymmetrically into the bifurcation.

Further features and advantages of the invention become apparent fromthe subsequent description of embodiments referring to the appendedFigures.

FIG. 1 illustrates a first embodiment of a sector antenna according tothe invention in an exploded view;

FIG. 2 is a perspective view of a second embodiment of the sectorantenna, in an assembled state;

FIG. 3 is a schematic view of half of a transversal hollow waveguide andchambers located thereat;

FIG. 4 is a schematic view of the coupling portion between alongitudinal hollow waveguide and a transversal hollow waveguide of thesector antenna;

FIG. 5 is an azimuth direction-characteristic of a antenna according tothe invention;

FIG. 6 is a diagram of the elevation direction characteristic of theantenna;

FIG. 7 is an exploded perspective view of a third embodiment of theantenna according to the invention; and

FIG. 8 is a top view of the plane of the first waveguide in the antennaof FIG. 7.

A first embodiment of the sector antenna of the invention is explainedreferring to FIG. 1. This Figure shows a plurality of metal plates 1 to7 from which the antenna is formed layer by layer. A plate 1 shown in abottom position in the Figure has a bore 8 and is provided forconnecting a coupling flange of a tubular hollow waveguide for feedingan RF signal to be transmitted by the antenna or for extracting an RFsignal received by it to the bottom side of the plate 1 at the bore 8.In the description, only the aspect of transmitting using the antennaaccording to the invention will be considered; it is understood,however, that the antenna can be used without modification for receivingan RF signal.

In a plate 2 arranged above plate 1, a first hollow waveguide, referredto as longitudinal hollow waveguide, extends in a longitudinaldirection. Via the opening 8, the first hollow waveguide is fed an RFsignal, which propagates inside the first longitudinal hollow waveguide9 from the bore 8 in opposite directions.

The first hollow waveguide 9 is formed as a slit extending over thecomplete height of plate 2.

At either side of the first hollow waveguide 9, flat grooves 10 extendin the longitudinal direction on top and bottom sides of plate 2.Together with the hollow waveguide 9, they delimit narrow surfaceportions 11 that are flush with the remainder of the top and bottomsides and are highlighted in the Figure by hatching and which carrysolder for soldering the plate 2 to the adjacent plates 1 and 3,respectively.

Plate 3 is a thin metal sheet which, when connected to plate 2, forms abroad sidewall of the rectangular longitudinal hollow waveguide 9. Aplurality of slit shaped excitation apertures 12 is formed in variousorientations with respect to the longitudinal direction of thelongitudinal hollow waveguide 9 and with various deviations with respectto the center plane of the longitudinal hollow waveguide 9.

In plate 4, a plurality of second hollow waveguides 12, referred to astransversal hollow waveguides, extends in a transversal direction of theplate, at right angles with the longitudinal hollow waveguide 9. Alltransversal hollow waveguides have a same length. An excitation aperture12 leads to each of these. Each transversal hollow waveguide 13 ispositioned such that the excitation aperture 12 leading to it is exactlyin the center of the transversal hollow waveguide 13. Therefore, thepositions of the transversal hollow waveguides 13 in the transversaldirection vary slightly, according to the various deviations of theexcitation apertures 12 leading to them.

Also in plate 4, portions 11 of upper and lower sides, which areintended to be coated with solder are separated from the remainder ofthe upper and lower sides by longitudinal grooves 10.

In a thin plate 5 to be soldered to plate 4, a plurality of couplingslits 14 is formed. The coupling slits 14 are oriented transversallywith respect to the transversal hollow waveguides 13 and are arranged ina matrix of lines and rows parallel to the transversal hollow waveguides13, one column of four coupling slits 14 being located above each of thetransversal hollow waveguides. Within a line, the positions of theindividual slits vary slightly in the transversal direction of plate 5,in correspondence with the varying positions in this direction of thetransversal hollow waveguides 13 themselves and the excitation apertures12, respectively.

A thick plate 6 to be placed on plate 5 has a plurality of through boresof approximately rectangular cross section, each of which forms achamber 15 together with the plate 5 and a plate 7 forming the outerside of the antenna. One coupling slit 14 of plate 5 and one sendingslit 16 of plate 7 leads to each of the chambers 15. The sending slits16 belonging to chambers 15 fed by a same hollow waveguide 13 arearranged at equal distances in a line. The individual lines are slightlydisplaced with respect to each other in the transversal direction ofplate 7.

In this embodiment, the thick plates 1, 2, 4, 6 may be formed bymachining from bulk material, whereas the thin plates 3, 5, 7 may bepunched from thin metal sheets, and the plates are connected to eachother by soldering.

In the embodiment shown in FIG. 2, the geometry of the hollow waveguidesand slits is not different from that of FIG. 1. It is formed of fourplates 1, 2′, 4′, 6′, wherein plate 1 corresponds to plate 1 of FIG. 1and plates 2′, 4′, 6′ may be regarded as one-part combinations of plates2 and 3, 4 and 5, 6 and 7, respectively, of FIG. 1.

Elements that are identical in the two embodiments have the samereference numerals in FIG. 2 as in FIG. 1 and are not described anew.FIG. 2 is a perspective view of the antenna, cut open along thelongitudinal hollow waveguide 11.

In order to be useable as a sector antenna for microwave applications,the direction characteristic of the antenna must meet the followingrequirements: In a first plane defined by the surface normal of plate 7and the transversal direction, referred to in the following as thehorizontal plane, the direction characteristic must have a main lobewhich is practically constant over an angular range of approximately90°, and no side lobes. In a plane referred to as the vertical plane,defined by the surface normal of plate 7 and the longitudinal direction,the direction characteristic must be sharply collimated and zero-free ina region close to the main lobe.

Considering the requirements for the direction characteristic in thehorizontal plane, it is sufficient to consider a single transversalhollow waveguide 13 and the chambers fed by it. The requirement of a 90°sector direction characteristic implies a distance of λ₀/2 betweenadjacent sending slits, wherein λ₀ is the free space wavelength of asignal to be radiated by the antenna. The relative amplitudes and phasesof the four sending slits 16 can be determined by a simulationcalculation. Since software for carrying out such calculations is known,no description thereof is necessary; in case of a 90° sector directioncharacteristic. The results obtained for the individual sending slits,one after the other, are:

-   -   (−5.7 dB; 122°); (0; 0); (0; 0); (−5.7 dB; 122°),        if the distance between the sending slits 16 is exactly 0.5 λ₀,        or    -   (−6.0 dB; 125°); (0; 0); (0; 0); (−6.0 dB; 125°),        for a distance of the sending slits of 0.52 λ₀.

In order to realize these amplitudes and phases, it is sufficient toplace the coupling slits between the chambers 15 and the transversalhollow waveguide 13 appropriately and to choose the length of thetransversal hollow waveguide 13 suitably, as explained in more detail inthe following.

FIG. 3 is a schematic view of a half of a transversal hollow waveguide13, bisected along its symmetry plane, and the chambers 15 located nearit, referred to as 15 a, 15 b in this Figure. As can be seen in thedrawing, there are three parameters which may be optimized for realizingthe desired phases and amplitudes: the distance l₁ between the symmetryplane and the coupling slit adjacent to it, here referred to byreference numeral 14 a, the distance l₂ between the coupling slit 14 aand the coupling slit 14 b adjacent to the short-circuited end of thehollow waveguide, and the distance l₃ between coupling slit 14 b and theend of the transversal hollow waveguide 13. These three parameters havebeen shown to be sufficient for realizing a 90° directioncharacteristic; in case of need, one might consider optimizing furtherparameters such as length and width of the coupling slits.

In order to find a distribution of the coupling slits 14 a, 14 b whichis suitable for realizing the desired sector direction characteristic,one may start from a combination of the parameters l₁, l₂, l₃ which inprinciple may be chosen arbitrarily, and the resulting distribution ofamplitudes and phases at the sending slits referred to as 16 a, 16 b maybe compared with the desired distribution and be optimized iteratively.

For l₃, it is suitable to take λ_(H)/2 as a starting value, whereinλ_(H) is the wavelength at the working frequency in the transversalhollow waveguide 13. By this selection, constructive interferencebetween a wave propagating towards the short-circuited end and a wavereflected from there is achieved, whereby the excitation of the chamber15 b and, hence, the amplitude at its sending slit 16 b, is maximum.

As a starting value of l₂,$1_{2} = {\frac{\Delta\quad\varphi}{2\pi}\lambda_{H}}$may be selected, wherein Δφ is the known desired phase differencebetween the sending slits 16 a, 16 b. In general, the phase differenceactually achieved with this starting value will differ from Δφ, sincethe positions of the coupling slits 14 a, 14 b at the bottom of chambers15 a, 15 b are not necessarily equal. In order to increase the actuallyresulting phase difference, l₂ will be increased and vice versa.

As a starting value of l₁, one may take e₁.

A direction characteristic obtained for parameter values l₁=0.25 λ_(H),l₂=0.30 λ_(H), l₃=0.53 λ_(H) is shown in FIG. 4. The curve H shows theamplitude for horizontal polarization normalized to maximum, and curve Vis the amplitude for vertical (cross) polarization. For horizontalpolarization, a 90° sector direction characteristic with a very smallripple between 0 and ±45° and a steady decrease to less than −35 dB at90° can be seen. The vertical radiation is nowhere more than −42 dB. Asteeper shape of the flanks of curve H might be obtained by increasingthe number of chambers 15.

By optimizing, l₁, l₂, l₃ are obtained as multiples of λ_(H). Since thehollow waveguide wavelength λ_(H) depends on the width a of the hollowwaveguide according to the formula${\lambda_{H} = \frac{\lambda_{0}}{\sqrt{1 - \left( \frac{\lambda_{o}}{2a} \right)^{2}}}},$it may become much longer than the free space wavelength λ₀ close to thecritical frequency. This might cause the coupling slits for the 14 a, 14b to be so far apart from each other along the transversal hollowwaveguide 13 that the chambers 15 a, 15 b cannot be located so that theyconnect the coupling slits 14 a, 14 b with the sending slits 16 as, 16 blocated at a distance λ_(H)/2. However, this problem may be avoided ifthe width a of the transversal hollow waveguide 13 is chosen largeenough. A width$a = \frac{\lambda_{0}}{2\sqrt{1 - \frac{\lambda_{0}^{2}}{4d^{2}}}}$equal to that of the longitudinal hollow waveguide has shown to beappropriate, it is also compatible with the requirement that thetransversal hollow waveguide 13 must not be wider than what correspondsto the distance d between excitation apertures 12.

While for the case of the 90° sector direction characteristic asconsidered up to now, for sending slits already provide a good result,for realizing a 45° sector, an arrangement of six sending slits is moreappropriate, since here a higher flank steepness of the directioncharacteristic is necessary. The required amplitudes and phases at thesending slits are calculated by simulation, as above; for the individualsending slits, one after the other, what is obtained is:

-   -   (−5.7 dB; 123°); (−5.65 dB; 76°), (0; 0); (0; 0); (−5.65 dB;        76°)(−5.7 dB; 123°).

The distances of the coupling slits among each other and between themand the end of the transversal hollow waveguide can be found iterativelyby optimization as described above.

In the vertical plane, a sharply collimated, zero-free radiationcharacteristic is desired. Here, too, simulation calculations accordingto known methods enable to calculate optimum amplitudes and phases forthis purpose for a plurality of sending slits placed at a verticaldistance d from each other. An example of an elevation directioncharacteristic with curves H, V for horizontal and verticalpolarizations, respectively, that can be realized with the group antennaaccording to the invention is shown in FIG. 6.

Since the dimensions of all transversal hollow waveguides 13 and thepositions of the excitation aperture 12 and the coupling openings 14 andthe chambers 15 connected thereto and their sending slits 16 is the sameat each transversal hollow waveguide 13, the phase difference betweenexcitation at the aperture 12 and radiation from the sending slits 16 isthe same. It is therefore sufficient to excite the transversal hollowwaveguides 13 with amplitudes and phases corresponding to these optimalrelative phases and amplitudes in order to obtain a corresponding phaserelationship between sending slits 16 located one above the other ofvarious transversal hollow waveguides 13. These amplitudes and phasesmay be tuned by appropriate choice of deviation e and rotation angle θof the slit-shaped excitation apertures 12 with respect to the centerplane 11 of the longitudinal hollow waveguide 9 (see FIG. 4).

A third embodiment of the antenna according to the invention is shown inan exploded view in FIG. 7. This embodiment, like that of FIG. 2, ismade up of four plates 1″, 2″, 4″, 6″. The plate 1″ differs from theplate 1 of FIGS. 1 and 2 merely by the position of the bore 8 which,here, is close to an edge of plate 1″.

In the plate 2″, a tree structure 20 is machined. A trunk 21 of the treestructure 20 is formed by a chamber to which, in an assembled state ofthe group antenna, the bore 8 leads. From this trunk 21, two mainbranches 22, 23 extend in opposite directions. These main branchesbifurcate repeatedly and finally end at excitation apertures 12, each ofwhich feeds a transversal hollow waveguide 13 in plate 6″. Theexcitation apertures are all congruent and aligned with each other.Mutually adjacent excitation apertures 12 are alternatingly connected tomain branches 22 and 23. The main branches 22, 23 bifurcate repeatedlyin order to reach the excitation apertures 12. The branches leading tothe excitation apertures 12 are formed of portions 24 extending inparallel to the direction of alignment of the excitation apertures 12,portions 25 that extend perpendicular to this direction, and T-shapedbifurcations 26, as can be seen detail in the top view of plate 2″ ofFIG. 8. With this structure, it is easy to design the tree structure 20such that due to different path lengths between the trunk 21 and thevarious excitation apertures 12, desired phase differences between theindividual excitation apertures 12 result. Consider e.g. the excitationapertures referred to as 12 a, 12 b in FIG. 8, which are supplied by acommon T-bifurcation 26 ab. A desired phase displacement between the tworesults from an appropriate choice of the length of portions 24 a, 24 b,i.e. from the placement of the T-bifurcation 26 ab in the verticaldirection of FIG. 8. In the same way, the phase relationship between theexcitation apertures 12 c, 12 d can be set by placing the T-bifurcation26 cd. The phase difference between the excitation apertures 12 a, 12 c,however, results from the position of a T-bifurcation 26 a-d feedingboth together. This method may be repeated cyclically, until finally, byplacing the trunk 21 in the horizontal direction of FIG. 8, the phaserelationship between the excitation apertures fed by main branch 22 andby main branch 23, respectively, is determined.

A tongue 27 extends into each T-bifurcation 26. This tongue determinesthe width of the passage between the portion 25 extending horizontallyin the Figure and the two vertical portions 24 of each T-bifurcation,and thus, the distribution of the amplitude of an incoming wave onto thetwo vertical portions 24.

The set of tongues 27 that are passed by a wave in a branch of the treestructure between the trunk 21 and an excitation aperture 12 defines theamplitude at this excitation aperture 12.

1-23. (canceled)
 24. A hollow waveguide group antenna, comprising: atransversal hollow waveguide extending in a first direction in space;and a plurality of chambers each having a sending/receiving slit andbeing coupled to the transversal hollow waveguide by a coupling slit,the sending/receiving slits being placed at a fixed distance, thecoupling slits being distributed in the first direction in space at thetransversal hollow waveguide differently from the sending/receivingslits such that a wave at a working frequency propagating in thetransversal hollow waveguide excites the sending/receiving slits withamplitudes and phases suitable for realizing a sector directioncharacteristic.
 25. The group antenna according to claim 24, in that thefixed distance is between 0.5 λ₀ and 0.65 λ₀, wherein λ₀ is a free spacewavelength of the wave at the working frequency of the group antenna.26. The group antenna according to claim 24, in that the coupling slitsand the sending/receiving slits are oriented transversally with respectto the first direction in space.
 27. The group antenna according toclaim 24, in that the transversal hollow waveguide has a short circuitat at least one end thereof.
 28. The group antenna according to claim27, in that the short circuit is spaced at a distance from the nextadjacent coupling slit, the distance being approximately half of ahollow waveguide wavelength of the wave at the working frequency. 29.The group antenna according to claim 28, in that the distance of theshort circuit from the next adjacent coupling slit is between 0.5 and0.55 times the hollow waveguide wavelength.
 30. The group antennaaccording to claim 24, in that the coupling slits are arranged mirrorsymmetric with respect to a symmetry plane extending transversally withrespect to the first direction in space, and in that the transversalhollow waveguide has an excitation aperture intersecting the symmetryplane.
 31. The group antenna according to claim 30; in that thetransversal hollow waveguide has a short circuit at both ends thereof.32. The group antenna according to claim 24, in that the coupling slitsare numbered between four and six.
 33. The group antenna according toclaim 30, in that the coupling slits number four, and in that two of thecoupling slits adjacent to the symmetry plane are located at a distancefrom the symmetry plane, the distance being one quarter of a hollowwaveguide wavelength of a wavelength at the working frequency.
 34. Thegroup antenna according to claim 30, in that the coupling slits numberfour, and in that one of the coupling slits adjacent to the symmetryplane is located at a distance from another coupling slit adjacent tothe short circuit, the distance being 0.3 times a hollow waveguidewavelength.
 35. The group antenna according to claim 24, and a pluralityof plates, the transversal hollow waveguide being formed in at least oneof the plates, and the chambers being formed in another of the plates.36. A two-dimensional group antenna, comprising: an assembly of hollowwaveguide group antennas, each including a transversal hollow waveguideextending in a first direction in space, and a plurality of chamberseach having a sending/receiving slit and being coupled to the respectivetransversal hollow waveguide by a coupling slit, the sending/receivingslits being placed at a fixed distance, the coupling slits beingdistributed in the first direction in space at the respectivetransversal hollow waveguide differently from the sending/receivingslits such that a wave at a working frequency propagating in therespective transversal hollow waveguide excites the sending/receivingslits with amplitudes and phases suitable for realizing a sectordirection characteristic; and the transversal hollow waveguides of theassembly being parallel to each other.
 37. The group antenna accordingto claim 36, in that each transversal hollow waveguide has an excitationaperture leading to a hollow waveguide common to several of thetransversal hollow waveguides.
 38. The group antenna according to claim37, in that the common hollow waveguide is a longitudinal hollowwaveguide extending linearly in a second direction in space.
 39. Thegroup antenna according to claim 38, in that the longitudinal hollowwaveguide is a rectangular hollow waveguide, and in that the excitationapertures are arranged in a side wall of the longitudinal hollowwaveguide having a width equal to λ₀ divided by two times the squareroot of one minus λ₀ squared divided by four times d squared, wherein λ₀is the free space wavelength of the working frequency, and d is thedistance between adjacent excitation apertures.
 40. The group antennaaccording to claim 38, in that the excitation apertures are slits, arotation angle of which defined with respect to the second direction inspace and/or a deviation thereof from a center of the longitudinalhollow waveguide being different for mutually adjacent excitationapertures.
 41. The group antenna according to claim 40, in that themutually adjacent excitation apertures have rotation angles anddeviations with opposite signs.
 42. The group antenna according to claim38, in that the common hollow waveguide has a tree structure with atrunk and a plurality of branches, each of which connects the trunk toone of the excitation apertures.
 43. The group antenna according toclaim 42, in that the tree structure has two main branches extendingfrom the trunk at opposite sides of a plane extending through theexcitation apertures, the excitation apertures of mutually adjacenttransversal hollow waveguides being connected to different ones of thesemain branches.
 44. The group antenna according to claim 43, in that thephases of a wave fed in at the trunk differ by not more than 2π at theexcitation apertures.
 45. The group antenna according to claim 40, inthat the slit shaped excitation apertures have a mean length of λ₀/2,wherein λ₀ is a free space wavelength at the working frequency of thegroup antenna.
 46. The group antenna according to claim 37, and aplurality of plates, wherein the common hollow waveguide is formed in aplate different from a plate for the transversal hollow waveguides andthe chambers.